Low noise tunnel-emission cathode



United States Patent 3,250,920 LOW NOISE TUNNEL-EMISSION CATHQDE Glen Wade, Wayland, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Sept. 21, 1962, Ser. No. 225,266 4 Claims. (Cl. 307-835) This invention relates generally to tunnel-emission devices and particularly to a low-noise device employing a tunnel-emission cathode, and more particularly to tunnelemission cathodes, involving a sandwhich made up of at least a first conducting body, a barrier material, and a second conducting body. A beam of electrons is emitted from the first conducting body. These electrons are believed to originate in the second conducting body and to tunnel through the barrier material into the first conducting body. Certain of these electrons will have enough energy to pass over the emission potential barrier into the space beyond. They may thenbe collected by an anode. The cathode is instant starting and promises long life and high current density.

The invention further relates to a tunnel-emission cathode in which electrical means are provided for controlling the magnitude of the noise temperature of the beam of emitted electrons. The noise temperature of the beam is not equal to the cathode temperature, as with thermionic emission, but depends on the value in volts of a built-in energy window for the emitted electrons. The noise temperature of the beam may be substantially lowered by maintaining the value of this energy window at a value on the order of a fraction of 2. volt as will be explained below.

A feature of this invention is that the temperature of the beam from the tunnel-emission cathode can be made extremely low, lower than any known beam-type device capable of operation in the microwave region. Useful current densities can be maintained.

These and other features of the invention will be better understood by reference to the following description taken in conjunction with the appended drawings wherein:

FIG. 1 is a diagrammatic illustration of an electron tube embodying a preferred form of the invention;

FIGS. 2a, 2b, and 2c are schematic diagrams of certain elements of the device of FIG. 1 prior to assembly, together with energy band diagrams thereof;

FIG. 3 is a schematic diagram and energy band picture of the assembled elements of the device of FIG. 1;

FIG. 4 is a schematic diagram and energy band picture of the elements of the device of FIG. 1 with voltage sources connected; and FIG. 5 is a side view of the device taken substantially on line 55 of FIG. 4.

With reference to FIG. 1, a tunnel-emission cathode indicated generally at and adapted for use in accordance with this invention produces an electron beam 11. For purposes of illustration, the cathode 10 is used in a wave-sustaining device, in this case a traveling-wave tube having an evacuated envelope indicated at 12, although it is to be understood that the tunnel-emission cathode 10 may be used generally wherever a thermionic cathode can be used.

The tunnel-emission cathode 10 is shown here as a three-element structure for purposes of illustration. The cathode 10 has a conducting body 13 in the form of a metallic substrate which acts as a source of electrons and which is separated from another conducting body 14, in the form of a metallic film, by a layer of barrier material 15. It will be understood; however, that the invention is not limited to a particular form of conducting bodies 13 and 14 or barrier material 15. It will also. be understood that the invention applies to tunnel-emission cathodes having any number of elements.

An electrical potential V is placed across the barrier material 15 by means of an adjustable voltage source 16 connected to the metallic substrate 13 and metallic film 14. A second voltage source 17 is connected between the metallic film 14 and the accelerating anode 18 of an otherwise conventional traveling wave tube so that the electron beam 11 is subjected to an electrical potential V A helical conductor 19, to which is connected a signal input 20 of microwave energy and a signal output 21 and is maintained at the potential of anode 18, encloses the beam 11 as is well known in the art. A collector 22 which is kept at the same potential as the anode 18 receives the electron beam 11 after its passage through the enclosed space of the helical conductor 19.

The operation of the tunnelemission cathode can be best understood by reference to FIGS. 2, 3, and 4. FIG. 2a shows the metallic substrate 13, and an energy band picture 23 for this element. FIG. 2b shows the barrier material 15 and an energy band picture 24'for this element. FIG. 20 shows the metallic film 14 and an energy band picture 25 for this element.

An energy band picture shows in diagrammatic form the distribution of filled and unfilled electronstates. The energy band pictures'23, 24, and 25 are shown for convenience at 0 K. at which temperature the conduction electrons in a metal are not all condensed into a state of zero energy as predicted by classical mechanics but rather, in accordance with quantum mechanical principles, they fill all allowed energy levels up to some 5 electronvolts above the ground state.

The level which divides the filled and vacant regions is known as the Fermi level at absolute zero and such levels are indicated by the straight horizontal lines 26 and 27 in FIG. 2. At absolute zero the Fermi level thus has the significance of a cut-off energy. With k being Boltzmanns constant and T being temperature, it will be understood that as the temperatures rises, levels within about kT below the Fermi level at absolute zero become partly depopulated and levels within about kT above the Fermi level at absolute zero become partly populated. This is indicated for the metallic substrate 13 by dotted line 28.

The Fermi level for an insulator or a semi-conductor does not have the significance of a cut-off energy since it is predicated on the basis of a free electron model which, though the excellent model for metals, is poor for insulators and non-conductors. However, the Fermi level is a useful concept even in this case, as will be seen below. The Fermi level for the barrier material 15, which may be an insulator, or a semiconductor operated at low temperatures,-is indicated at 29 and may be, for example,

midway between the conduction band 30 and the valence band 31 of the energy band picture 24. The band 32 in between the bands 30 and 31 is known as the forbidden band since electrons cannot have energies in that band. At absolute zero, the valence band 31 is filled and the conduction band 30 is unfilled.

The lines 33 and 34 in the energy band pictures 23 and 25 indicate the energy level above which electrons can escape from the surface of the metallic substrate 13 and the metallic emitter 14 respectively. The position of these lines 33 and 34 which are here called emission levels will vary with the conditions of the space outside the elements 13 and 14. Thus the value of the emission level 33 will be different for a vacuum and for a gaseous atmosphere as is well known. The energy separation between the emission level 34 and the Fermi level 27 for the metallic emitter 14 is the work-function for that material.

FIG. 3 shows the metallic substrate 13, the barrier material 15 and the metallic film 14 of FIG. 2 located in contiguous contact with one another. The energy band picture 35 shows that the Fermi level for these three elements is a constant, assuming thermal equilibrium. For convenience of illustration, the metallic substrate 13 and the metallic film 14 have been assumed to have substantially identical Fermi levels 26 and 27 which Will be true, for example, if they are made from the same material. The barrier material 15 has also been assumed to have its Fermi level 29 before contact with the metallic substrate 13 and the metallic film 14 also at the same energy. If this were not so, the Fermi levels 26, 27, and 29 after the elements are in physical contact would still be constant but band bending the interfaces of the material would occur. This fact is not of inherent significance in this discussion. Hence, for simplicity, it is to be assumed here that band bending does not take place. Since the metallic substrate 13 and the metallic film 14 are of the same material, their emission levels 33 and 34 in the absence of special preparation will be at the same energy.

FIG. 4 shows the cathode of FIG. 3 having connected thereto the adjustable voltage source 16, voltage source 17, and anode 18 as shown in FIG. 1. The effect of this can be seen by energy band picture 37. In order to compensate for the potential applied by source 16, electrons flow across the barrier material with the result that the Fermi level 27 for the metallic film 14 is lowered by an amount V below the Fermi level 26 of the metallic substrate 13. The Fermi level 29 of the material 15 slopes to connect to both Fermi levels 26 and 27. The emission level 34 of the metallic film 14 is also lowered byV so that it is a potential difference 6 below the Fermi level 26 of the metallic substrate 13. The edge of the conduction band 30 next to the metallic emitter 14 is also lowered V energy units. Line 38 indicates the anode potential, and the dotted line 28 indicates depopulation and population of the energy levels as discussed above in connection with FIG. 1.

It will be understood that as long as 6 is a positive quantity, there will be some electrons in the metallic substrate 13 which have higher energies than the emission level 34 of the metallic film 14, since as explained above, at absolute zero all electron levels in the metallic substrate 13 are filled up to the Fermi level 26.

However, 6 is within the forbidden band 32 of the material 15. Therefore, electrons from the metallic substrate 13 must tunnel through the forbidden band 32 in order to reach the metallic film 14. Quantum mechanics predicts a finite probability for this tunneling which has been proved by experiment.

Electrons in the substrate 13 which have an energy range between the Fermi level 26 and 6 units below the Fermi level 26, can thus escape from the surface of the metallic film 14 and be drawn to the anode 18. The larger 6 is, the more electrons can be drawn off, but these electrons will have energies ranging from the energy of the Fermi level 26 to an energy which is 6 energy units below the Fermi level 26. Thus larger 6s give larger energy distributions. 6 is therefore a measure of the size of an energy window and is controlled by the adjustable voltage source 16.

The inventor has found that the temperature of the beam of emitted electrons is given substantially by T =27306(volts) when T =0 K. Here T is the beam temperature in K. and T is the cathode temperature in K. If 6 is 0.1 volt, T is 273 K. If 6 is 0.01 volt, T is 273 K. which represents a very low-noise beam. This result contrasts sharply with the case of thermionic emission where T is approximately equal to T Since it is required that in order to provide a low noise beam the cathode must be cooled to a predetermined temperature, this may be accomplished by closely encircling the metal substrate 13 (FIG. 1) with a coil 42 of tubing through which a selected coolant may be directed from a supply source. Coolant such as liquid nitrogen or liquid helium, for example, will flow through the coil when impelled by any suitable pumping means (not shown), whereby the cathode structure will be cooled by conduction, the heat being carried away by the coolant. Thus, the cathode may be cooled to Within any desired low temperature range depending upon the particular materials being used. In the foregoing examples, the preferred temperature of the cathode (T is 0 K. The purpose of cooling is to stabilize the electron energies in the metal substrate 13 to virtually those levels below the Fermi level, as stated above.

The effect of merely cooling the three elements 13, 14, and 15 does not result in a low-noise beam as would be expected in the prior art. It is necessary that 6, the size of the energy window, be small in order to have a lownoise beam.

The effect of cooling, once the magnitude of 6 has been adjusted, can be understood by reference to the dotted line 28. As explained above with reference to FIG. 1, at elevated temperatures, T of the three elements 13, 14, and 15 there will be electrons having energies above the Fermi level 26. Since these electrons also have energies above the emission level 34, of the metallic film 14 some of them will also be emitted through the surface of the metallic film 14.

The increased spread in energies will increase the noise temperature T of the beam. Therefore, in a preferred embodiment of the invention, the temperature T is kept low. However, it is to be understood that lowering T has little effect if 6 is not properly adjusted as explained above.

The emission-current density varies approximately as the second power of 6. Thus, lowering the noise temperature also lowers the emitted current density. However,

reasonably useful current densities can still be maintained at low-noise temperatures.

FIG. 5 shows the surface 39 of the metallic film 14 with a typical position for the electrode 40 which is connected to the adjustable voltage source 16 of FIG. 4. Because the electrode 40 is localized, the Fermi level 27 in FIG. 4 for the metallic film :14 will not be uniformly lowered throughout the film -14.

This is due to the fact that there is a lateral leakage current flow of electrons which tunnel through the barrier material 15 with energies below the emission level 34 of FIG. 4. This current must pass through the voltage source 16 of FIG. 4. Since the thickness of the metallic film 14 should be small compared to a mean free path, in order to allow for electron transmission into the vacuum, the resistance it offers to this leakage current is large with a resultant potential drop along the metallic emitter surface as indicated at 38 in FIG. 4. This means that at point 41, for example, 6 will have a different value from that at point 42.

Since the noise temperature of the beam depends sensitively on the magnitude of 6, any-variation in 6 will affect the noise. Because of the dependence of the current density on 6 variations 'Will also lead to nonuniform emission. Means must therefore be provided so that this variation is reduced. This is accomplished in accordance with this invention by the use of a superconducting material for the metallic film 14. The superconducting feature has the effect of assuring a constant potential over the entire surface 39 even though the electrode 40 applies the potential at only one point. Thus a uniform 6 over the entire surface 39 is assured.

For optimum results, the materials for the metallic substrate 13, the barrier material 15 and the metallic film 14 should be chosen so that the conduction band 30 of the barrier material 15 in FIG. 4 is always higher than the Fermi level 26 in the metal substrate 13 in FIG. 4. This will be true in the case in which the energy gap in the forbidden region 32 of the barrier material in FIG. 4 is preferably somewhat more than twice the work function, 5, of the metallic film 14. This ensures that the electronswhich have tunneled through from the metallic substrate 13 will not lose energy by entering the conduction band 30 of the barrier material 15 in FIG. 4 and as a result, fall below the emission level 34 of the metallic film 14 and therefore fail to be emitted. The current density remains high even with a small 6.

It can be seen that the barrier material 15 may be either an insulator or a semiconductor. The metallic substrate 13 and the metallic film 14 may be the same material or may be difiFerent. The metallic film 14 may be treated, for example, with potassium or rubidium to lower the emission level 34 in FIG. 4 and make it more uniform throughout the material of the film 14.

A typical assembly might consist of gold for the metallic substrate 13 and the metallic film 14 and aluminum oxide for the barrier material 15. These three elements might be built up by evaporating first gold on a suitable surface such as glass to obtain the-metallic substrate 13, then evaporating aluminum, followed by anodizing the aluminum to obtain aluminum oxide as the material 15 followed by a second evaporation of gold to obtain the metallic 14. The film 14 might be several hundred Angstroms thick. Such a thickness would be sufiiciently thin to prevent the electrons losing energy by collisions in traveling through the film 14.

Other suitable materials would be aluminum oxide as the barrier material 15. Or the oxides of beryllium, tantalum or titanium could be used as the barrier material 15 in combination with platinum or palladium as the metallic substrate -=13 and metallic film 14.

From the foregoing it will be seen that all of the objects of this invention have been achieved by the novel device shown and. described. However, it is to be understood that modifications of the device and in its mode of operation may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims.

What is claimed is:

1. In a tunnel-emission cathode having at least a first conducting body, a second conducting body, and a barrier material positioned between said first body and said second body;

means for producing a beam of electrons from said cathode;

and means for maintaining said beam at a low noise temperature, said last named means comprising;

means for maintaining the emission level of said first body at a potential diiference below the Fermi level of said second body which is of a value in accordance with the equation where T is said noise temperature in degrees Kelvin, and 5 is said potential difference in volts; and means for maintaining the temperature of the cathode at a level where the electron energies in said second body are stabilized to substantially those levels below the Fermi level. 2. In a tunnel-emission cathode having at least a first conducting body, a second conducting body, and a barrier material positioned between said first body and said second body;

means for producing a beam of electrons from said second body into said first body;

and means for controlling the noise temperature of said beam, said last named means comprising;

means for maintaining the emission level of said first body at a controlled potential difference below the Fermi level of said second body;

and maintaining the temperature thereof at a level Where the electron energies in said second body are stabilized to substantially those levels below the Fermi level.

3. In a tunnel-emission cathode having at least a first conducting body, a second conducting body, and a barrier bmaterial positioned between said first body and said second means for producing a beam of electrons from said second body into said first body;

means for controlling temperature of said beam, said last named means comprising;

means for maintaining the emission level of said first body at a potential difference below the Fermi level of said second body;

means for controlling the magnitude of said potential difference;

means for maintaining the conduction band of said barrier material above said Fermi level of said second body;

and means for maintaining the temperature of the cathode at a level where the electron energies in said second body are stabilized to substantially those levels below the Fermi level.

4. In a wave sustaining device employing a tunnel emission cathode having at least a first conducting body, a second conducting body, and a barrier material positioned between said first body and said second body;

means for producing a beam of electrons from said second body into said first body; and means for controlling the noise temperature of said beam, said last named means comprising;

means for maintaining the emission level of said first body at a potential difference below the Fermi level of said second body; and means 'for maintaining the temperature of the cathode at a level where the electron energies in said second body are stabilized to substantially those levels below the Fermi level.

References Cited by the Examiner UNITED STATES PATENTS 3,056,073 9/1962 Mead 317-234 GEORGE N. WESTBY, Primary Examiner.

V. LAFRANCHI, R. JUDD, Assistant Examiners. 

1. IN A TUNNEL-EMISSION CATHODE HAVING AT LEAST A FIRST CONDUCTING BODY, A SECOND CONDUCTING BODY, AND A BARRIER MATERIAL POSITIONED BETWEEN SAID FIRST BODY AND SAID SECOND BODY; MEANS FOR PRODUCING A BEAM OF ELECTRONS FROM SAID CATHODE; AND MEANS FOR MAINTAINING SAID BEAM AT A LOW NOISE TEMPERATURE, SAID LAST NAMED MEANS COMPRISING; MEANS FOR MAINTIANING THE EMISSION LEVEL OF SAID FIRST BODY AT A POTENTIAL DIFFERENCE BELOW THE FERMI LEVEL OF SAID SECOND BODY WHICH IS OF A VALUE IN ACCORDANCE WITH THE EQUATION 